The advantages of using a comb filter for providing luminance and chrominance signal separation in a television receiver are quite well known. A conventional comb filter includes a delay device for imparting a one horizontal line (1-H) delay to the video input signal, an adder for adding the delayed and non-delayed input signals to provide a luminance output signal and a subtractor for subtracting the delayed and non-delayed signals to provide a chrominance output signal.
The delay element used in a comb filter has a very significant effect on the comb filter frequency response characteristic. The ideal delay element would have a constant delay independent of frequency thereby providing a linear phase response and thus a uniform amplitude response throughout the video frequency band to be filtered. The delay element presently closest to this ideal is the digital delay line employing binary storage elements. Unfortunately, digital delay lines are costly and relatively complex because video signals are conventionally received in analog form and so conversion to digital form is necessary to use digital delay lines of the binary storage type.
Analog delay elements, such as glass delay lines, are conventionally used in comb filters as a means of reducing the cost and complexity of the filter. Comb filters employing glass delay elements (or other analog delay elements) are however subject to delay variations with frequency (i.e., non-linear phase response) and thus the overall amplitude response of such comb filters tends to vary with frequency.
Manufacturers of comb filters of the glass delay line type have recognized this problem and have taken steps to correct for undesired amplitude variations which, to a certain extent, minimizes the problem. As an example, amplitude response 2 of FIG. 2 (PRIOR ART) herein illustrates the uncorrected response of a commercially available comb filter employing a glass delay line. As shown, the uncorrected response exhibits a rising response from about 2.5 MHz to 4.5 MHz with a peak of about +6 dB. The manufacture of this filter has included a correction circuit having the characteristic shown by response 3. The resultant corrected response is illustrated by curve 4. As seen the amplitude corrected comb filtered signal (4) exhibits a dip of about 2 dB at a frequency of about 2.5 MHz, it returns to zero dB at about 3 MHz and decreases thereafter. As shown, the rather large 6 dB variation of the uncorrected comb filter has been reduced to only 2 dB by internal correction circuits provided by the manufacturer (TDK Incorporated, model HCF0020C).
The comb filter correction (to 2 dB) provided by the filter manufacturer is quite good considering the rather large (+6 dB) variations typical of uncompensated comb filters with glass delay lines. It has been recognized by the present inventor, however, that it would be desirable to provide a further correction of the comb filter response to reduce the luminance signal variations to a few tenths of a decibel in the luminance frequency range. To this end the present inventor has previously designed a receiver in which an auxiliary luminance amplitude correction circuit was included which achieved this goal. This receiver has been manufactured and sold under the brand name "RCA" in receivers employing chassis identification numbers CTC 148/149 and is shown herein as FIG. 1 (PRIOR ART). An understanding of this prior art receiver (10) is helpful to appreciating the subtle nature of the present invention and so this receiver will now be described in detail.
Receiver 10 includes an RF processor 12 having an antenna or cable input terminal 14 for receiving an RF video input signal S1. Processor 12 includes conventional elements such as a tuner for selecting channels, an IF amplifier and sound trap and a video detector for providing a demodulated (baseband) composite video output signal S2 at its output. A video switch 14 is provided for selecting signal S2 or an auxiliary composite video baseband signal S3 applied to an auxiliary input terminal 16. The selected signal S4 provided by video switch 14 is applied to a comb filter 18 of the type employing an analog type delay element (e.g., a glass delay line). The filter 18 exhibits uncompensated amplitude variations as a function of frequency and has a luminance signal output 20 for providing a luminance output signal Yl and a chrominance signal output 22 for providing a chrominance output signal C1.
The chrominance signal C1 is applied to a first input 24 of a luminance-chrominance (Y-C) processor 26. The luminance signal Yl is applied to a second (luminance) input 28 of processor 26 by means of a cascade connection comprising, in the exact order named of: a contrast control 30, a luminance/chrominance delay correction circuit 32 having source (R1) and termination (R2) resistors and a comb filter correction circuit 34 (outlined in phantom).
The function of contrast control 30 is to control the peak-to-peak amplitude of the comb filtered luminance signal. In the prior art receiver this control is provided by a 300 Ohm potentiometer coupled in series with a 430 Ohm resistor, the latter serving as a minimum limit to the potentiometer output. The function of luma delay 32 is to delay the comb filtered luminance signal by about 300 nano-seconds. This compensates for difference in luminance and chrominance signal delay in the receiver due to their different bandwidths and thus provides proper registration of the luminance and chrominance signals for display on display 36. The function of the comb filter correction circuit 34 is to correct uncompensated errors in the amplitude response of comb filter 18. This correction is shown in FIG. 2 by curve 5 which has a +2 dB peak at a frequency of about 2.5 MHz. As a result the uncorrected comb filter output (response 4) is flattened to the response 6 which is essentially constant up to a frequency of about 4.2 MHz, the full extent of the luma band in the NTSC standard. Processor 26 includes conventional circuits for providing functions such as chroma demodulation, hue and saturation control and matrixing for providing a suitable output regional (e.g., RGB) for display by display unit 36.
The correction circuit 34 (outlined in phantom) comprises an input 40 connected to the output 38 of luma delay unit 32 (e.g., 300 nanoseconds) and an output 42 connected to input 28 of processor 26. Internally unit 34 includes a parallel resonant circuit comprising an inductor L1 and a capacitor C1 connected in common to terminals 40 and 42 at one end thereof and coupled to ground at the other end thereof by means of a resistor R3. Representative values for L1, C1 and R3 are 100 micro-Henrys, 39 pico-Farads and 8200 Ohms, respectively. This circuit provides the luma signal boost of about 2 dB shown in FIG. 2 as response 5 which flattens the overall response 6 to within less than 1 dB over the full luma band as shown.